1. Field of the Invention
The present invention relates to voltage monitoring circuits, and in particular, to voltage monitoring circuits for rechargeable batteries used in low voltage applications.
2. Description of the Related Art
Voltage monitoring circuits play a key role in controlling the charging and discharging of rechargeable battery cells, such as lithium and lithium-ion battery cells used in portable electronic equipment such as portable computers. As is well known, lithium and lithium-ion secondary (i.e., rechargeable) battery cells require protection from prolonged over-charging and over-discharging so as to prevent degradation of the performance of the cells, as well as to prevent ruptures in the casing of the cells and combustion of the electrolyte within the cells.
In order to accomplish this, protection circuits, typically internal to the battery pack, monitor the individual cell voltages for over-charge and over-discharge conditions. Such circuits monitor the individual cell voltages to determine if a maximum or minimum cell voltage threshold has been crossed. For safety reasons, resistors are connected in series between the monitored cell terminal and the power and sensing terminals of the monitoring circuit. Such resistors are intended to limit any current that would flow from the battery cell in the event that a short circuit developed within the monitoring circuit.
Referring to FIG. 1, two terminals are typically used to connect the monitoring circuit to the monitored terminal of the battery cell. One terminal is used for sensing the cell voltage, while the other terminal is used for providing power to the monitoring circuit. Accordingly, six terminal connections are required: a voltage sensing connection for the positive cell terminal; a power connection to the positive cell terminal; a ground connection to the negative cell terminal; discharge and charge control terminal connections to the series-connected MOSFET switches; and a sensing connection at the negative terminal of the battery pack.
However, the use of such series resistors pose a number of problems. First, such resistors dissipate power which could otherwise be used more productively within the monitoring circuit itself. Second, since two resistors are required so that the voltage sensing circuit can make a more accurate measurement of the cell voltage without an offset introduced by current flowing through its resistor, an extra terminal is required for providing power via the power supply terminal (VDD) within the monitoring circuit.
One possible solution is the elimination of the safety resistors R1, R2. This would allow one terminal to be used for both power and sensing with minimal error. However, due to the aforementioned safety concerns, this is generally an unacceptable compromise.
Another possible solution is to attempt to correct for the offset introduced across the safety resistor due to the voltage drop caused by the power current. However, maintaining accuracy of such an offset correction is virtually impossible unless the magnitude of the power current can be held constant and predictable. Such consistency in power current, however, is virtually impossible due to variations caused by changes in operating modes, temperature, supply voltage and process variations. Hence, this solution is generally impractical and, therefore, also unacceptable.
Yet another possible solution is to design the voltage monitoring circuit such that its power current magnitude is so low as to generate an insignificant voltage drop across the safety resistor. However, circuits capable of operating in this manner result in internal resistive elements which are large in area and value, and with such low current values errors in accuracy are introduced by noise and leakage effects.
Referring to FIG. 2, one conventional technique samples the cell voltage via resistor R2 with a capacitive voltage divider. Initially switches S1 and S2 are in positions 1, thereby discharging capacitors C1 and C2. During sampling, switches S1 and S2 are in positions 2 and the divided cell voltage VS across capacitor C1 is compared in voltage comparator A1 with a reference voltage VREF generated by the reference voltage generator. Voltage comparator A1 and the reference voltage generator are powered by the battery cell via resistor R1.
Referring to FIG. 3, another conventional technique effectively merges the reference voltage generation and voltage comparison functions. The sampled voltage VS is buffered by buffer amplifier A2 and is used to drive a band gap voltage generator circuit. The resulting voltages V1, V2 are compared in voltage comparator A3 (with hysteresis). The resulting voltage comparison signal Vc is then used by the remainder of the control circuit (not shown) for generating the charge and discharge control signals for the MOSFET switches MC, MD. (Examples of such control circuits, many of which are well known in the art, can be found in U.S. patent application Ser. No. 08/801,162, filed Feb. 18, 1997, and entitled "Methods and Apparatus for Protecting Battery Cells from Overcharge," and in U.S. patent application Ser. No. 08/904,138, filed Jul. 31, 1997, and entitled "Bidirectional Current Control Circuit Suitable for Controlling the Charging and Discharging of Rechargeable Battery Cells, " the disclosures of which are incorporated herein by reference.)
Accordingly, it would be desirable to have a technique by which accurate voltage monitoring can be performed with fewer battery cell connections required, while maintaining safeguards against potential short circuit conditions.